As the quality of indoor air becomes less predictable as far as air pollutants are concerned and as more and more respiratory diseases are caused by airborne particles of a smaller and smaller size the demand increases for domestic air cleaners that are capable of removing particles in the sub micron range at high removal efficiencies.
Filtration media capable of high filtration efficiencies for sub micron particles have been in wide use in "clean-room" applications and other hospital and industrial applications. Media classified as HEPA are characterized by their ability to filter up to 99.97% of all airborne particles of a size of 0.3 micron and larger.
The HEPA media comprises a dense non-woven sheet of glass fibers held together with an organic binder. Due to its dense structure it causes a high pressure-drop in the air passing through it. The high pressure difference between up stream and down stream on the filter will cause a significant amount of unfiltered air to flow through any gap or opening that might exist around the filter or within the air cleaner structure propper.
Since HEPA filters arrest up to 99.97% of all particles, even the smallest amount of unfiltered air mixing with the clean air will bring the filtering efficiency percentage down significantly.
It is for that reason that it is of paramount importance to insure during the design of high efficiency air cleaners that the sealing of the filter is optimized and that no unfiltered air is permitted to infiltrate into the air cleaner, and bypass the filter.
Owing to the high pressure drop caused by the media and owing to the fact that the filter will build up over time a layer of trapped dust and pollutants which will further increase the pressure drop, it is imperative to provide each air cleaner with a filter of a large through-flow area. This will reduce the face velocity and, accordingly, result in a reduced pressure drop. As an example:--at a face velocity of 10.5 feet per minute the pressure drop across a typical HEPA-filter is 1.25" W.G., while tripling the face or through-flow area the velocity will be reduced to 3.5 Ft/min and the pressure drop to about 0.4" W.G.
A filter area three times as large would naturally last three times as long before necessitating replacement. The necessity to enlarge the filter area without enlarging the overall face area of the filter assembly has led to the solution of folding the woven filter material into relatively small, closely spaced pleats. In order to prevent the pleats from collapsing one against the other the distance between pleats is secured by spacers as e.g strings or strips coated with an adhesive spanned across the entire filter.
Being faced with the problem to incorporate a pleated filter unit in a portable air cleaner leaves the designer with several options as how to incorporate an exchangeable filter unit of large through flow area and a blower unit in an attractive housing.
The most straight forward filter configuration would be that of a pleated HEPA-filter configured into a flat structure. The advantages are simple construction, but with the inherent drawback that the air cleaner unit becomes very large in its frontal dimensions, and that the design imposes serious restrictions on the manner in which air is drawn into the blower inlet or inlets.
A better approach to solving the problem of a small face area is described in U.S. Pat. No. 4,629,482 (G. B. Davis) which discloses a portable air purifier having a generally cylindrical air filter mounted to receive incoming air through its peripheral surface sucked in by a vertical single inlet blower and delivered to the outside through circumferential outlet openings. The design is advantageous in that it simplifies sealing of the filter in the cylindrical housing, but has a number of serious drawbacks, e.g.:--the design does not permit the use of a double-inlet blower which necessitates provision of a blower of larger diameter and consequently a larger housing. And the fact that the air outlets are near the floor prevents proper air circulation through the room.
The present invention has as its main object to accommodate a number of needs associated with the design of a portable air cleaner with high filtering efficiencies by incorporating a pleated HEPA-filter unit.
It is a further object to provide an air cleaner that is compact, effects a good air circulation through out the room and offers ready access to the filter for replacing it by the user.
It is another object of the invention to provide the air cleaner with a double-inlet blower with a view to reducing the inlet velocity into the blower and to obtaining higher blower efficiency.
It is a further object of the invention to provide for a compact air cleaner where the blower housing in partially located inside the filter structure.
It is another object of the invention to provide for perfect sealing between the filter structure and the air cleaner housing.
It is still another object of the invention to provide for simple effective sealing around the blower outlet by molding the blower housing out of a semi pliable material.
It is yet another object of the invention to provide for low cost means to sense and signal that a filter should be replaced.
Additional objects of the invention will become evident from the following figures and as the air cleaner is described in detail.